Create target_traits
* Start to wrap target system in type_traits-like interface
* Derive target_is_<system> from std::conditional's type
+ Add missing target_is_<system> templates
* Rename templates to target_system_is_<system>
+ Add target_compiler_is_<compiler> templates
* Make target_system_is_unknown account for misuse
* Make target_compiler_is_unknown account for misuse
+ Add target_cpu_is_<cpu>
- Remove old code
Merge branch 'target_traits'